This invention concerns the field of electrical energy transducer apparatus i.e., electrical motor and generator machines, especially of the switched reluctance motor/generator type, which may be useful in demanding environments such as in an aircraft. The invention is concerned with the maintenance of partial operation in such apparatus under component failure conditions.
In the design of military aircraft an American philosophy has historically considered crew safety and the return to base under extremes of battle damage items of design priority. A striking example of this philosophy is found in World War II equipment wherein the piston and propeller driven P-38, P-47 and F-51 tactical aircraft were sometimes placed at speed, maneuverability and range disadvantages relative to directly encountered concurrent era enemy aircraft--at least partly because of armor plate and redundant systems incorporated into the American planes. Many who followed the air war in Europe and the South Pacific are also familiar with stories of bomber aircraft incorporating this philosophy limping home with disabled engines, electrical interruptions and missing control surface as a result of battle damage, a return enabled at least in part by observation of this return to home philosophy in the aircraft's design. A perhaps extreme example of the opposite to this philosophy may be found in the practice of omitting the weight and space penalties attending even a parachute in some of the missions and the enemy aircraft used in this era.
A considerable measure of this philosophy has survived into the present day and into the design of jet propelled aircraft and their subsystems. In particular considering the heavy reliance on electronic systems in a present day aircraft there has been an ongoing effort to provide many military aircraft with a reliable and battle hardened source or sources of electrical energy. In multiple engine aircraft this effort is often manifest in the form of plural electrical energy sources, e.g., alternators or direct current generators energized by separate propulsion engines of the aircraft. Further extensions of this multiple source thinking may include the addition of multiple alternators or generators coupled to a single one of the propulsion engines and the addition of auxiliary power sources driven by a separate prime mover such as a small automatic-starting turbine engine. Parenthetically speaking, the present inventors recognize the formal distinction between the alternator and generator electrical sources recited in the foregoing discussion. For brevity and convenience purposes however, in the following description material this formal distinction is not rigidly observed and the term generator is, for example, used in a generic sense to refer to either machine. In another sense, this blurring of distinction can be rationalized in view of possible question as to the correct generic name for an alternator machine which includes an integral rectifier diode array and provides direct current output energy.
For the jet engine propelled aircraft of the future there is considerable interest in continuing this philosophy in an electrical machine that is directly integrated into the engine assembly. In such an arrangement the machine rotor is carried on the main compressor turbine shaft of the engine for example and is therefore driven without use of gearing which usually involves bearings and lubricants and other complexities. Additionally in such an arrangement the same machine can act as both an electrical motor for engine starting and as the source of aircraft electrical energy once the engine is running. The switched reluctance motor/generator electrical machine is found particularly desirable for such combination service since it is capable of high energy density operation, is free of brushes and their finite life characteristic and free of rotating windings and their support structure. Such a machine is capable of high revolution rates and provides additional performance advantages in the areas of reliability, weight and physical size. A generic aircraft propulsion engine arrangement of this type is shown in FIG. 8 of the drawings herein.
According to this present day thinking the switched reluctance motor/generator can be made part of an aircraft electrical system operating at considerably greater voltages than the twenty eight volts of direct current used in many present day aircraft electrical systems, i.e., a system operating at voltages in the range of two hundred seventy volts of direct current is considered feasible. The reduced amount of copper needed for electrical bus conductors is found to be attractive particularly from the aircraft weight penalty viewpoint with such operating voltage levels. The use of alternator type machines coupled to semiconductor rectifier or electrical switching devices are additional parts of this thinking.
According to an additional aspect of this contemplated future aircraft electrical system, it is desirable for such a switched reluctance motor/generator to be provided with a plurality of electrically independent winding circuits or winding channels, winding channels which may be operated each independent of the other with separate electronic regulation if needed or which may be connected in a combination such as a paralleled channel arrangement for a normal and fill utilization of the machine's energy transducer capacity. The electrically independent machine winding channels moreover may each be of a multiple winding phases nature. For example a three channel, three phase machine having a number of different rotor and stator pole configurations can be utilized according to this philosophy. One such machine configuration which appears especially attractive for this and for present invention purposes for example is comprised of a three channel, three phase eighteen pole stator and a twelve pole winding-free rotor. A machine of this nature is in fact disclosed in the later detailed description topic of the present document. It may be appreciated that a machine of this electrically independent winding circuits or winding channels type offers significant advantages when disposed within the propulsion engine of an aircraft or in other limited space locations since fitting the functionally equivalent plural separate machines in such locations, in order to obtain redundant electrical energy sources, could be difficult.
A primary purpose for using such a multiple winding channel, multiple phased switched reluctance motor/generator machine in a military or other aircraft resides in the above referred-to desire for reliability and redundancy in the aircraft--to provide crew protection and a maximally assured return to home base (after a mission involving battle damaged or inadvertent electrical energy source failure). When this damage occurs directly to the aircraft generator windings or when damage to other parts of the aircraft electrical system result in overload damage to the windings of a generator machine channel it is nevertheless desirable that the aircraft and its crew have some less than normal but adequate source of electrical energy available to operate the aircraft and its systems in returning home. Prior to the present invention such reduced operation of a switched reluctance motor/generator machine has been difficult because attempts to use remaining undamaged parts of the machine have inherently resulted in continued involvement of damaged portions of the machine, i.e., because the machines in question could not be divided into truly electrically isolated and magnetically isolated winding channels.
There is therefore perceived to be needed in the field of aircraft electrical machines, and in electrical machines used in other applications, a machine such as a multiple channeled switched reluctance motor/generator in which effective and substantially complete electrical isolation between electrical winding portions of the machine can be accomplished. Moreover it is desirable for such isolation to be accomplished in a manner which can be called into service quickly and preferably without human election--and preferably accomplished in a manner which provides an uninterrupted source of electrical energy. The present invention is believed to provide a viable answer to these needs.
The U.S. patent art indicates the presence of significant inventive activity in the area of electrical machines having at least general background relationship with machines made according to the present invention. Included in this patent art is the alternator electrical machine and alternator switching arrangement of U.S. Pat. No. 5,198,972 issued to LaFuze, a patent also assigned to the United States Air Force. In the LaFuze patent, semiconductor switching devices are connected to the output of a three phase alternator machine in a novel alternating current output phase switching arrangement. The invention is not however concerned with a reluctance motor/generator apparatus nor with the problem of maintaining at least partial operation in a damaged alternator system.
The U.S. Pat. No. 5,703,421 of Durkin is also of general background interest with respect to the present invention. This patent discloses a reluctance motor/generator machine possibly also suitable for aircraft use and for the combination duty of electrical energy source and engine starting in the aircraft. In the U.S. Pat. No. 5,703,421 patent, novel arrangements for extracting thermal energy from within the rotor and from the machine stator enclosure of the reluctance motor/generator are disclosed. The U.S. Pat. No. 5,703,421 patent is not however concerned with partial operation of a reluctance motor/generator machine under element failure conditions or with winding switching arrangements used to facilitate this partial operation as are involved in the present invention.